1. Field of the Invention
The present invention relates to active electromagnetic interference (EMI) filters for motor drives, and more particularly to active EMI filters which employ magnetoresistive bridge circuits to provide very high bandwidth yet are small enough to be easily and conveniently incorporated into integrated circuit (IC) chips in motor drive systems.
2. Relevant Art
Modern motor drives generally utilize pulse width modulation (PWM) switched inverters to generate motor drive currents having precisely controlled characteristics. FIG. 1 shows a simplified schematic diagram of a conventional three-phase PWM motor drive, generally denoted at 10. Here, incoming three-phase A.C. power is provided to a rectifier circuit 12 which provides D.C. to an inverter 14 on buses 16 and 18. The inverter, in turn, provides controlled three-phase A.C. to motor 20.
Numerous inverter designs exist, but these generally incorporate pairs of power semiconductor switches (not shown) such as metal oxide semiconductor field effect transistors (MOSFETS) or insulated gate bipolar transistors (IGBTS) for each motor phase. These are switched on and off by pulse width modulated gate control signals provided by a PWM control circuit 22 which operates in response to current feedback signals from the motor and a speed setpoint signal to vary the duty cycles of the inverter transistors. This technology is well known to persons skilled in the art, and further description will be omitted in the interest of brevity.
One significant problem with PWM inverter motor drives is electromagnetic interference (EMI) in the form of conducted and radiated noise due to the rapid on-off switching of the inverter transistors. With continuing advances in power semiconductor technology, it has become possible to increase to the switching frequencies and the rate of change in the voltage of the inverter transistors. This has the advantage of reducing the acoustic noise of the motors, but the increased switching speed worsens the EMI problem.
To deal with this, EMI filters are conventionally incorporated in motor drives. Initially, these were passive devices employing of inductors and capacitors. In practice, however, in order to provide adequate filtering, large and bulky filter inductors are required. As a consequence, active EMI filters employing current sensors and transistor amplifiers are now customarily employed.
FIG. 2 shows a simplified schematic diagram of a motor drive 30 incorporating an active EMI filter. Here, the active EMI filter 32 is inserted between rectifier 12 and inverter 14. In this circuit, the filter topology is based on so-called feed-forward cancellation and employs complementary transistor switches 32 and 34, a common-mode current sensor 36 including a current transformer (CT) 38 and a sense amplifier 40 to drive the transistor switches. CT 38 is constructed of a core 42 formed of a ferrite material or the like, coupled to the high and low side D.C. buses 16 and 18 of rectifier 12. An output is provided to sense amplifier 40 by a winding 44. Basically, the filter 36 generates a signal which matches and therefore cancels the common mode noise current. This basic concept is also well known. Devices of this kind are shown, for example in the related U.S. patent applications referred to above.
Several different types of current sensor can theoretically be used in an active EMI filter, but in practice, the CT has been the device of choice for sensing common mode noise current because it conveniently provides current-to-current transformation, and galvanic isolation.
EMI can be a serious problem due to the wide adoption of PWM motor drives, and electromagnetic noise compliance standards presently call for effective noise cancellation for conductive common mode emission noise over a frequency range of 150 kHz to 30 MHz. The ability of a CT to meet such requirements depends on the material used for the core. However, to provide a wide range of frequency filtering operation and to accomplish effective cancellation against very fast spike noise current (less than one microsecond), the core must have wide frequency range of transformation characteristics.
The CT also must provide a high fidelity (i.e., low distortion) output to faithfully reproduce fast noise current waveforms. Therefore the core must have good permeability. However, these two performance criteria contradict each other for any soft magnetic material, as a wide frequency range requires reduced permeability of the core material. Thus there must be a trade-off s between linearity (permeability) and frequency range.
Attempts to satisfy this trade-off generally involve use of large cores in an effort to sustain transformation fidelity over a wide frequency operating range. However, when large noise currents are encountered, as in high horse power motor drive systems, the size of the CT becomes large enough that it poses a practical implementation limit. Also, the manual assembly steps associated with the wire windings become an issue and will alos introduce inaccuracy in the curent transformer ratio. Thus, a need still exists for improvement in the current sensors used in active EMI filters. The present invention seeks to satisfy that need.
It is accordingly an object of the present invention to provide an improved active EMI filter for PWM motor drives.
A further object of this invention is to provide an improved current sensor for active EMI filters which provides very high bandwidth signal transformation in a configuration which permits incorporation in an IC device.
These objects are accomplished according to this invention by providing a common mode current sensor for an active EMI filter in the form of a magneto-resistive (MR) bridge.
Certain nickel-iron alloys such as Permalloy (Ni81Fe19) are known to be magnetoresistive, i.e., to exhibit electrical resistance which depends on the strength and direction of nearby magnetic fields, and it has been proposed to use such devices as current sensors by combining several sensor elements in a Wheatstone bridge mounted in proximity to a current-carrying bus or a current trace on a semiconductor substrate. A reference voltage is applied to the bridge and voltage output, which changes due to the magnetically induced resistance changes, is measured as an indication of an incident magnetic field.
Magnetoresistive sensors of this type are disclosed and claimed in commonly assigned U.S. patent application Ser. No. 10/228,881, filed Aug. 26, 2002 in the name of Jay Goetz, entitled MAGNETORESTSTIVE MAGNETIC FIELD SENSORS. The disclosure of this application is incorporated by reference herein, as if fully set forth.
According to the present invention, it has been found that a properly configured MR device can be used as a common mode current sensor in an active EMI filter. This permits the entire filter to be constructed as an IC device with the MR sensor magnetically coupled to an internal current carrying structure which is connected externally between the rectifier and the inverter in the motor drive.
The sensor is connected to the D.C. output buses of the rectifier 12 so the D.C. flows through two internal lead frame bars of the IC device. However, the D.C. current flows in the same direction through both lead frame bars, while the common node noise current flows in appropriate directions through the buses. As a result, the sensor is responsive only to the common node noise current.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.